fix: use anchor paid_at as canonical double-claim guard in claim_prize

The PrizeClaimed flag (fast-path) can TTL-expire while winner rows
stay live from get_winners/winner_at reads.  Added && paid_at.is_none()
to the anchor scan so a claimed anchor (paid_at: Some(...)) prevents
replay even if the separate flag has expired.

Author: Shadow-MMN

contracts/events/src/event_ops.rs

@@ -736,7 +736,7 @@ pub fn claim_prize(
         if w.recipient != recipient || w.position != position {
             continue;
         }
-        if w.milestone.is_none() {
+        if w.milestone.is_none() && w.paid_at.is_none() {
             winner_info = Some((idx, w.amount, w.reputation_bump));
             break;
         }